In Eq. (5.11), DS 1 is the entropy change of uncross-linked polymer chains, and
DS 2 is correlated with the entropy change caused by the formation of a cross-linked
chain pair.
5.3 Nature of Cross-Linking Leading to the Formation
of Polymer Gels
The arrangement and interactions between polymer components are key factors to
determine the performance of gel materials. The predesigned interactive groups in
gelator molecules enable direct connections between the gelators via non-covalent
or dynamic covalent interactions. The dynamic feature of these bonding schemes
often results in the dynamic properties of polymer gels such as self-healing, and
responsive or adaptive to external environments.
5.3.1 Hydrogen-Bonded Polymer Gels
Hydrogen bonding is an electromagnetic attractive interaction between hydrogen
bond donor and acceptor atoms. A typical hydrogen bond donor consists of a
hydrogen atom attached to an electronegative atom, therefore forming a dipole in
which the hydrogen atom is slightly positively charged. In contrast, an atom as a
hydrogen bond acceptor is highly electron withdrawing, for example, nitrogen,
oxygen or fluorine. Cross-linker bonded by reversible hydrogen bonding can
potentially enable supramolecular polymer gels with dynamic properties, such as
stimuli-responsiveness, self-healing and post-synthesis processability.
In the research area of hydrogen-bonded gels, an important sub-division is the
design of polymer networks with sufficient mechanical toughness. Tough synthetic
polymer gels, particularly hydrogels, are good candidates for biomedical applications. Examples include scaffolds for tissue engineering [24], optical and fluidic
actuators [33] and carriers for drug delivery [34]. The formation of highly tough
polymer gels can be achieved by tuning the molecular structure of building blocks.
Among the polymer backbones used for gel preparation, poly(ethylene glycol)
(PEG) shows hydrophilicity and melting behaviour under physiologically relevant
temperatures [35]. Guo and co-workers have prepared a PEG-based polymer gel
containing ureidopyrimidinone (UPy) moieties within the backbone (Fig. 5.4a)
[36]. As shown in the inset of Fig. 5.4b, a fourfold hydrogen-bonding dimer is
formed between two complementary UPy moieties. Driven by this strong interaction between polymer chains, the polymers first organize into a dry semicrystalline
morphology. Upon water encapsulation, the bulk PEG-based polymer networks are
converted into a supramolecular hydrogel. Experimental results on the mechanical
properties show that the hydrogel exhibits significant strength recovery even at a
162
5 Polymer Gels
DS 2 is correlated with the entropy change caused by the formation of a cross-linked
chain pair.
5.3 Nature of Cross-Linking Leading to the Formation
of Polymer Gels
The arrangement and interactions between polymer components are key factors to
determine the performance of gel materials. The predesigned interactive groups in
gelator molecules enable direct connections between the gelators via non-covalent
or dynamic covalent interactions. The dynamic feature of these bonding schemes
often results in the dynamic properties of polymer gels such as self-healing, and
responsive or adaptive to external environments.
5.3.1 Hydrogen-Bonded Polymer Gels
Hydrogen bonding is an electromagnetic attractive interaction between hydrogen
bond donor and acceptor atoms. A typical hydrogen bond donor consists of a
hydrogen atom attached to an electronegative atom, therefore forming a dipole in
which the hydrogen atom is slightly positively charged. In contrast, an atom as a
hydrogen bond acceptor is highly electron withdrawing, for example, nitrogen,
oxygen or fluorine. Cross-linker bonded by reversible hydrogen bonding can
potentially enable supramolecular polymer gels with dynamic properties, such as
stimuli-responsiveness, self-healing and post-synthesis processability.
In the research area of hydrogen-bonded gels, an important sub-division is the
design of polymer networks with sufficient mechanical toughness. Tough synthetic
polymer gels, particularly hydrogels, are good candidates for biomedical applications. Examples include scaffolds for tissue engineering [24], optical and fluidic
actuators [33] and carriers for drug delivery [34]. The formation of highly tough
polymer gels can be achieved by tuning the molecular structure of building blocks.
Among the polymer backbones used for gel preparation, poly(ethylene glycol)
(PEG) shows hydrophilicity and melting behaviour under physiologically relevant
temperatures [35]. Guo and co-workers have prepared a PEG-based polymer gel
containing ureidopyrimidinone (UPy) moieties within the backbone (Fig. 5.4a)
[36]. As shown in the inset of Fig. 5.4b, a fourfold hydrogen-bonding dimer is
formed between two complementary UPy moieties. Driven by this strong interaction between polymer chains, the polymers first organize into a dry semicrystalline
morphology. Upon water encapsulation, the bulk PEG-based polymer networks are
converted into a supramolecular hydrogel. Experimental results on the mechanical
properties show that the hydrogel exhibits significant strength recovery even at a
162
5 Polymer Gels